1. Field of the Invention
The present invention relates to an optical communications system using optical wavelength multiplexing technology.
2. Description of the Related Art
Recently, active research and development are made to establish a multimedia communications network which integrally accommodates various types of data for use not only in the conventional voice communications, but also in image data communications using a computer, etc. Especially, a capacity required to display one screen of image data is larger than for voice data. To realize a TV telephony, etc., a large capacity of image data must be transferred in real time. Therefore, a communications network to realize such multimedia communications requires a very high data transmission speed. Under this situation, a broadband ISDN, etc. has been studied for implementation. However, since the transmission speed is limited in the electric communications, the optical communications are being highlighted. Particularly, in the optical communications, an optical wavelength division multiplexing system can accommodate a larger volume of information, and is desired to be realized for practical use.
In response to a strong request to largely increase the volume of data to be accommodated, recently optical wavelength multiplexing systems tend to be operated with an increasing number of multiplexed wavelengths, up to 32, 64, and 128 waves. However, the maximum number of wavelengths originally designed for a communications system is not normally used from the start of its operation. The number of multiplexed wavelengths is increased depending on conditions such as an increasing number of subscribers, etc. As a result, it is necessary to design the system such that the number of applicable wavelengths can be sequentially increased on demand for a line without using all applicable wavelengths from the beginning.
When the number of wavelengths to be multiplexed is increased, the number should be sequentially increased without affecting the wavelengths already in service. This is referred to as an in-service upgrade.
In an optical wavelength multiplexing system using an optical amplifier in a transmission line, the optical amplifier is normally operated under ALC (automatic level control) or APC (automatic power control). In this case, the optical amplifier is operated such that the value of an output level can be a predetermined value regardless of an input level (represented by {output per wave (dBm)}+10 log n, where n indicates the number of input wavelengths).
The optical wavelength multiplexing system is designed in a way that the system can optimally function when all wavelengths are input. However, in the optical wavelength multiplexing system, all wavelengths are not utilized from the beginning. Normally, the number of wavelengths are sequentially increased on demand.
If the number of input wavelengths is small, the optical amplifier excessively amplifies the level of a signal light, and a too high level distorts the signal light due to a nonlinear effect, thereby deteriorating the transmission quality.
When the number of multiplexed wavelengths is 4 through 8 and a small number of upgrading operations are actually performed, a dummy light source is provided, to avoid the above described problem, even for a wavelength not in service to transmit a light having the number of wavelengths necessary for the optimum operation in the conventional technology.
FIG. 1 shows the conventional method.
In FIG. 1, the optical wavelength multiplexing system is designed to set the maximum number of multiplexed wavelengths to 4. When the system is established, only a 2-wave optical signal is used in a line actually in service as shown in FIG. 2. In FIG. 2, the wavelengths xcex of 1 and 2 are used. In this case, since the system is designed such that it can optimally function when the number of multiplexed wavelengths are four, the transmission quality is deteriorated by the operation of the optical amplifier as described above if only two waves are used. Therefore, dummy lights are transmitted as the wavelengths 3 and 4 to maintain the transmission quality of the system. Normally, the dummy lights contain no information, so are transmitted to enable an optical sender to maintain the transmission quality.
However, in the above described method, it is necessary to first provide a relatively expensive optical sender (OS), thereby causing a problem of costly initial investment. Furthermore, the larger the number of multiplexed wavelengths, the larger the number of optical senders required to maintain the transmission quality, thereby further causing demerit to the method.
As described above, when the system is operated with the number of multiplexed wavelengths smaller than the number of multiplexed wavelengths with which the system optimally functions in the optical wavelength multiplexing system, the transmission quality is deteriorated if no action is taken, thereby failing to provide satisfactory services.
Furthermore, when the number of dummy lights is equal to the number of unused wavelengths as in the conventional technology, the initial investment can be reduced with a smaller number of optical senders for transmitting dummy lights if the maximum number of multiplexed wavelengths is small. However, the number of optical senders for transmitting dummy lights becomes large if a system having a large maximum number of multiplexed wavelengths is designed on demand, thereby causing a problem of very large initial investment.
An object of the present invention is to provide a device for preventing the transmission quality of the optical wavelength multiplexing system from being deteriorated with the initial investment suppressed even if the number of multiplexed wavelengths is small.
The optical wavelength multiplexing system according to the present invention includes an optical signal transmission unit, corresponding to at least one wavelength, for transmitting an optical signal containing information to be transmitted; and a control light transmission unit for multiplexing with the light signal a light having a wavelength different from the wavelength of the optical signal containing the information, transmitting the multiplexing result, and adjusting the optical level of a total of the actually transmitted optical signal and the light such that it is substantially equal to the total optical level of a signal transmitted when an optical signal having the maximum number of multiplexed wavelengths which can be accommodated in the optical wavelength multiplexing system is transmitted.
In an optical wavelength multiplexing system, the terminal station according to the present invention includes an optical signal transmission unit, corresponding to at least one wavelength, for transmitting an optical signal containing information to be transmitted; and a control light transmission unit for multiplexing with the light signal a light having a wavelength different from the wavelength of the optical signal containing the information, transmitting the multiplexing result, and adjusting the optical level of a total of the actually transmitted optical signal and the light such that it is substantially equal to the total optical level of a signal transmitted when an optical signal having the maximum number of multiplexed wavelengths which can be accommodated in the optical wavelength multiplexing system is transmitted.
According to the present invention, it is not necessary to stop an optical signal of a line currently in use when the number of multiplexed wavelengths is increased. The number of multiplexed wavelengths can be increased only by newly multiplexing an optical signal having a new wavelength, and reducing the level of a light transmitted from a control light transmission unit. Therefore, the in-service upgrade can be realized furthermore easily.
Additionally, unlike the conventional technology, the initial investment can be considerably reduced because it is not necessary to include a number of optical senders in the initial configuration.